Superconducting materials, in suitably developed forms, and at cryogenic temperatures, can transport without overheating, many times (in excess of 10 fold, and up to 100,000 fold) the electrical current that can be practically and economically transported by resistive materials such as copper, aluminum and silver of the same cross sectional area. For the purpose of clarity in this document, electrical conductors with cross-sectional shape aspect of less than about 3 are referred to as wires, while conductors with cross-sectional shape aspect greater than about 3 are referred to as tapes, and bundles comprised of two or more conductors are referred to as cables.
All three types can be used to produce coils that can generate very large magnetic fields, in stationary magnet applications like MRI, NMR and accelerator magnets, as well as in moving magnet applications such as in for example wind generators. However round wires and other conductors with low aspect cross sectional shapes, provide for superior, much less complex, more versatile cabling and coiling; simplified ease of use and lower cost. To date, however, and in spite of considerable effort, no high current density HTS oxide based superconductor has been commercialized in round wire or low aspect form due to difficulties in producing the well developed 2223/silver (also known as Bi2Sr2Ca2Cu3Ox/Ag where Bi2Sr2Ca2Cu3Ox is 2223 and Ag is silver) based superconductor and the 2G YBCO 123 based superconductors in any form other than as thin tapes with shape aspect ratios in excess of 10, and difficulties in developing scalable, high current density forms of 2212/silver (also known as Bi2Sr2Ca1Cu2Ox/Ag where Bi2Sr2Ca1Cu2Ox is 2212 and Ag is silver) wires and processes for their manufacture.
Recently however, considerable, rapid progress has been made on the development of 2212/Ag round and rectangular wire with critical current densities exceeding 800 A/mm2 at 4.2 K in a 5T field. These wires, coupled with embodiments of the present invention, enable the production of a practical, low aspect 2212 based wire with critical current density exceeding 500 A/mm2 at 4.2K and in greater than 5T field and with tensile stress tolerance exceeding 400 MPa, tensile strain tolerance exceeding 0.3% and bend tolerance allowing bending below a diameter of about 275 times the wire cross section in the bend direction, and preferably 250 times the wire diameter.
HTS based superconductors can also be used to transmit very large amounts of electric power in very high current cables over large distances with very little energy loss. In the case of magnets, the interaction between the moving charge in the conductor and large magnetic field can result in very large axial forces (the Lorentz force F=IL×B where I is electrical current, L is conductor length and B is magnetic field impinging on the conductor, and × denotes cross product) in the conductor at high fields, requiring reinforced conductors with very high levels of axial stress tolerance without degradation in conductor properties, primarily its current carrying capacity.
Although superconducting materials can transport much greater current densities than resistive materials, beyond a certain critical current (Ic) and corresponding critical current density level (Jc), they exhibit rapidly increasing resistance, limiting their use to the regime below Ic and Jc. Addition of reinforcement and other materials increases conductor cross sectional area, and therefore reduces the maximum engineering current density (Je) that the conductor can be operated at before reaching the intrinsic Jc of the superconducting material. It is therefore of great value and importance to add as little material as possible to attain a desired level of reinforcement (and insulation).
The 2212/Ag round and low aspect rectangular wire forms currently under development are the best candidates for achieving low field-ramp loss, high-current cables and HTS coils beyond the cabling, loss and current density limits of commercially developed high aspect YBCO 2G and 2223/Ag tapes. However, round, and low aspect rectangular 2212/Ag composite wire is currently inadequate for use in demanding coil and cable designs because of poor stress, strain, bend and indent tolerance.
The 880 C bake for achieving high current anneals the oxide dispersion strengthened Ag matrix, which combined with the low modulus of silver impairs limits tensile stress tolerance to below 200 MPa, tensile strain tolerance to below 0.3% and bend tolerance to diameters that are greater than 250 times the wire cross-section in the bend direction. In fact, 2212/Ag use is now limited to “wind and react”, where precursor wire/cable with ceramic insulation is wound into coils first, then baked at 880 C. This approach to coil and cable making severely limits the types of coils that can be produced, and it greatly increases the complexity and difficulty of making coils and cables.
These issues also challenged 2223/Ag high aspect tape product development in efforts to make them suitable for use in mechanically demanding applications. However, reinforcement techniques have been developed to solder laminate, in the state of the art, high strength stainless steel strips on each side of the tape, thereby increasing the tensile stress and bend tolerance of for example reinforced 2223/Ag tapes by more than two fold without impractically increasing reinforced tape cross sectional area compared to the core 2223/Ag cross sectional area. However no design or method has been advanced for improving the mechanical properties of round and low aspect 2212/Ag wires to the above described levels for enabling practical application of low cost, versatile react and wind approach to coil production with 2212/Ag based HTS conductors.
A significant part of the reason why round wire reinforcement has not occurred is that it has proven to be more difficult to reinforce the round and low aspect conductor shapes than the reinforcement of the thin, high aspect tapes. The low aspect conductors place more extreme demands on any approach to reinforce them adequately, in particular, uniformly, without adding so much material and thereby reducing conductor current density to obviate its utility, and without damaging the ceramic 2212 or 2223 superconducting material and thereby impairing its functionality.
Applications and aspects of embodiments provided herein address long-felt needs in the art. Stress, strain and bend tolerance herein is meant as retaining in excess of 95% of the original critical current level of the reinforced wire, after the independent application of any of the above described mechanical conditions anywhere along the wire's length. The mechanical condition where irreversible Ic degradation first exceeds 5% (and sometimes a low as 1% depending on the test equipment and method) is commonly measured by a transport 4 point test method.
In specific embodiments provided herein, voltage measuring electrical contacts are positioned on the reinforced superconductor inside of the region between electric current injecting contacts. Tensile stress and strain in the material is progressively increased while critical current is measured at each stress increment by sweeping current up to the onset of voltage. By this method the stress and strain resulting in the onset of irreversible Ic degradation in the superconducting material is determined at stress and strain conditions ranging from 300 K to 4 K, depending on the capabilities of the equipment.